In recent years, a liquid crystal display device, for example, has been widely used in a liquid crystal television, a monitor, a mobile phone, and the like as a flat panel display having characteristics, such as being thinner, lighter, and the like, than a conventional cathode ray tube television. As such liquid crystal display devices, those using an active matrix substrate are known. In such a liquid crystal display device, a plurality of data wirings (source wirings) and a plurality of scan wirings (gate wirings) are arranged in a matrix. In the proximity of an intersection of a data wiring and a scan wiring, a switching element such as a thin film transistor (TFT, hereinafter referred to as “TFT”) or the like and a pixel having a pixel electrode that is connected to the switching element are arranged in a matrix.
In the aforementioned active matrix substrate, other than the aforementioned TFT for driving a pixel, a TFT for a peripheral circuit is integrally provided. Furthermore, when an active matrix substrate is used in a liquid crystal display device that is equipped with a touch panel, a liquid crystal display device that is equipped with an illuminance sensor (ambient sensor), or the like, there has been suggested that in addition to the aforementioned TFTs for driving a pixel and for a peripheral circuit, a photodiode (thin film diode: TFD) be provided in the active matrix substrate. Thus, the active matrix substrate is a semiconductor device that is equipped with a plurality of thin film transistors and a photodiode.
Furthermore, in the active matrix substrate, typically, TFTs having mutually different configurations are used as TFTs for driving a pixel and for a peripheral circuit.
Specifically, as a TFT for driving a pixel, a TFT having a very low OFF leakage current is used. In a liquid crystal display device, a voltage applied to liquid crystal needs to be sustained during one frame period until the screen is updated. This is because, in a liquid crystal display device, if the OFF current (OFF leakage current) of the TFT for driving a pixel is high, the voltage applied to liquid crystal decreases over time, causing a risk of display characteristics deterioration. As a result, as the TFT for driving a pixel, an n-channel type TFT, for example, having an LDD configuration in which a low-concentration impurity region (LDD region: Lightly Doped Drain) is formed at least either between a channel region and a source region of the TFT or between the channel region and a drain region, is typically used. In this LDD configuration, an LDD region having a higher resistance than the source region and the drain region is provided between an edge of a gate electrode and the source region having a low resistance and between the edge and the drain region. This way, when the aforementioned LDD configuration is used, the OFF leakage current can be reduced significantly compared to a so-called single drain configuration TFT, which does not have an LDD region.
On the other hand, as a TFT for a peripheral circuit, a TFT having a high current driving power, i.e., a high ON current, is used. Specifically, as the TFT for a peripheral circuit, an n-channel type TFT having a GOLD (Gate Overlapped LDD) configuration, for example, is used. In this GOLD configuration TFT, a gate electrode overlaps an LDD region. Therefore, when a voltage is applied to the gate electrode, electrons that become carriers are accumulated in the LDD region overlapped by the gate electrode. As a result, in the aforementioned GOLD configuration TFT, the resistance of the LDD region can be reduced. Thus, lowering of current driving power of the TFT can be suppressed, and the ON current can be increased.
Alternatively, in the active matrix substrate, an n-channel type TFT and a p-channel type TFT having the aforementioned single drain configuration are also used as TFTs for a peripheral circuit.
As a conventional method of manufacturing a semiconductor device, as discussed in Japanese Patent Application Laid-Open Publication No. 2005-328088, for example, there has been proposed that TFTs corresponding to various circuits of an active matrix substrate be formed on a single substrate. Specifically, in this conventional method of manufacturing a semiconductor device, an n-channel type TFT for driving a pixel having the LDD configuration, a p-channel type TFT for a peripheral circuit having the single drain configuration, and an n-channel type TFT for a peripheral circuit having either the LDD configuration or the GOLD configuration are formed on a single substrate.
Furthermore, as a conventional method of manufacturing a semiconductor device, as discussed in WO 2008/133162 pamphlet, for example, there has been proposed that an active matrix substrate in which a photodiode and a TFT are monolithically provided be formed. In addition, the WO 2008/133162 pamphlet shows a photodiode formed of a PIN diode that is equipped with a lateral configuration. Furthermore, according to the conventional method of manufacturing a semiconductor device, variation in length of an intrinsic semiconductor region in a forward direction (i.e., channel length) can be suppressed by providing two metal wires above the intrinsic semiconductor region (i-layer). As a result, variation in output characteristics of the photodiode can be also suppressed.